1. Field of the Invention
The present invention relates to a memory updating system for a field device, used at a process field, that rewrites data, such as firmware or configuration data, stored in a nonvolatile memory in the field device.
2. Description of the Related Art
JP-A-2001-350504 and JP-A-2003-510710 are referred to as related art of the memory updating system.
JP-A-2001-350504 discloses the steps of: performing a calculation process in a first memory area; deactivating the first memory area; and activating a second memory area, whereby reconfiguration can be performed in an online mode.
JP-A-2003-510710 discloses the steps of: employing a standard communication protocol to download a program instruction from a host to a field device; and storing the program instruction, whereby re-programming of a field device in a process control network is performed.
FIG. 1 is a diagram showing the general configuration of a process control system. A plurality of field devices 9 are connected to a fieldbus 8 such as a FOUNDATION Fieldbus or a serial transmission fieldbus. Each of the field devices 9 incorporates an analog input function block AI or a PID function block PID.
The fieldbus 8 is connected to a host system 5 through an I/O unit 7 and a control bus 6. In such a process control system, a signal is received from a sensor (not shown), and data are transferred between the field devices 9 or between the field devices 9 and the host system 5.
In the field device 9, a program for performing a computation in order to convert a signal received from a sensor into an output signal, and configuration data indicating network information are generally stored in a nonvolatile memory, such as an EPROM, a Flash memory or a EEPROM.
Recently, in order to enhance a function decentralization for the purpose of process improvement, an additional function, such as an extension of a self diagnosis function, is also required to the field device 9. Furthermore, since there is the variety of functions, the possibility that bugs will exist in the firmware is also increased. Therefore, the updating of the firmware inside the field device 9 is also performed by using the communication function of the field bus 8.
In this case, a binary image of the firmware is transmitted to the host system 5, and is transferred, through the control bus 6, the I/O unit 7 and the field bus 8, to the field device 9, which then rewrites the received binary image. The binary image of the firmware may be obtained from a web site across the Internet.
FIG. 2 is a diagram showing a part of the Software Download specification of the FOUNDATION Fieldbus. This specification is based on an assumption that the firmware of the field device 9 will be updated with using a fieldbus communication, and commands such as PREPARE_FOR_DWNLD and CANCEL_DWNLD are prepared.
However, the field device used to update the firmware has the following problems.
A field device that employs a Flash ROM or an EEPROM as the nonvolatile memory can update the firmware through the fieldbus. However, current consumption inside of the field device temporarily increases during the update of the nonvolatile memory.
FIG. 3 is a drawing showing the actual value of the measured power consumed when data in the Flash ROM are rewritten. When data are rewritten in the Flash ROM, data in each sector are erased and then new data is written to the Flash ROM. The values IDL (I Download), which indicates an increase in the power consumed during sector erasing, are a maximum of 20 mA and an actual measurement of 10 mA. Further, the increase in the power consumption for data writing was the maximum of 20 mA, while the actual measurement was 1 mA. In accordance with these measurements, it is found that mainly during sector erasing the consumption of power is considerably increased.
Therefore, when the host system 5 arbitrarily rewrites the firmware stored in the field device 9, an increase of 20 mA in the consumption of power for each field device must be considered when designing a power source circuit and other components. Consequently, the costs will be increased.
When the host system 5 updates the firmware for the field devices 9 one by one, the consumption of power can be reduced to 20 mA at the maximum. However, because of a limitation in the transfer rate (31.25 kbps) for the fieldbus 8, about thirty minutes, for example, is required to update each device. When ten field devices 9 are constituted in one segment, a maximum of 300 minutes is taken in order to update all of the field devices 9, so that this is not practical.
Assuming that the firmware size is about 300 kilo bytes, the communication interval is one second and the number of bytes transferred during each communication interval is about 200 bytes, the following updating time is required for rewriting.
                              Updating          ⁢                                          ⁢          time                =                ⁢                  ROM          ⁢                                          ⁢          capacity          ×          Communication          ⁢                                          ⁢          interval                                                ⁢                                            (              seconds              )                        /            The                    ⁢                                          ⁢          number          ⁢                                          ⁢          of          ⁢                                          ⁢          transferred          ⁢                                          ⁢          bytes                                        =                ⁢                  300          ⁢                                          ⁢          k          ×                      1            /            200                                                  =                ⁢                              1500            ⁢                                                  ⁢            seconds                    =                      25            ⁢                                                  ⁢            minutes                              
Thus, since the power supplied to the field device 9 would become insufficient, data could not be rewritten, and the number of field devices 9 that could be connected to the fieldbus 8 is limited.